PROJECT SUMMARY/ABSTRACT Blood pressure (BP) based on measurements obtained in the clinic often does not accurately reflect a person?s BP outside of the clinic setting. Masked hypertension refers to BP levels not in the hypertensive range when measured in the clinic but in the hypertensive range when measured outside of the clinic. Previous studies, primarily from Europe and Asia, have reported masked hypertension to be common and associated with a two- times greater risk of cardiovascular disease (CVD) compared with non-hypertensive BP both in and outside of the clinic. The 2017 American College of Cardiology (ACC) / American Heart Association (AHA) BP guideline recommends screening for masked hypertension to guide the initiation and intensification of antihypertensive medication. Ambulatory blood pressure monitoring (ABPM), the reference standard for measuring BP outside of the clinic setting, measures BP over a 24-hour period including while a person is awake and asleep. However, the 2017 ACC/AHA BP guideline recommends only to use awake BP and not asleep or 24-hour BP to define masked hypertension, and BP thresholds for defining masked hypertension are lower in this guideline compared to prior definitions. We propose to: (1) determine the impact of using awake, asleep, and 24-hour BP versus awake BP alone on the prevalence of masked hypertension and associations with sub-clinical CVD and albuminuria; (2) determine the impact of the 2017 ACC/AHA BP guideline definition on the prevalence of masked hypertension and associations with sub-clinical CVD and albuminuria; and (3) develop a prediction model to identify who should be screened for masked hypertension. We will address these aims among participants taking and not taking antihypertensive medication, separately, and for non-Hispanic whites, non- Hispanic blacks and Hispanics. To address these aims, we will pool previously collected data from 5 NHLBI- funded population- and community-based studies: the Jackson Heart Study, the Coronary Artery Risk Development in Young Adults Study, the Masked Hypertension Study, the Improving the Detection of Hypertension Study, and the North Carolina Masked Hypertension Study. These studies include non-Hispanic whites (n=1,202), non-Hispanic African Americans (n=1,612), Hispanics (n=359), and other race/ethnicity groups (n=126) who had their BP measured in the clinic and by ABPM; completed an echocardiogram and an albuminuria assessment; and have extensive covariate data for analysis. Of the 3,299 participants (>2,800 with clinic SBP<140 mm Hg and <90 mm Hg) we plan to include in our analyses, the mean age was 48 [95% confidence interval: 43 to 53 years of age] years, 62% were female, and 74% were not taking antihypertensive medication. This research could lead to updated guideline recommendations to define masked hypertension and an algorithm to identify who should undergo ABPM to screen for masked hypertension. Findings could also improve assessment of BP-related CVD risk and identify adults who would benefit from treatment.